Water level is not water depth
Is a river's water level the same as its depth?
No. A level reading says how high the water surface stands above the gauge's zero point. It does not say how deep the water is at any spot in the channel.
On this page
The short answer
A level reading is a height. It is counted up from the zero point of the gauge, so it tells you where the water surface is. Depth is a distance down from the surface to the bed, at one spot. They are two different measurements.
To know the depth at a spot, you must know where the bed lies there. The reading alone cannot tell you.
Why the bed matters
The bed lies at a different height at different spots along and across the channel, and it moves. The US Geological Survey notes that streams erode and become deeper, or deposit sediment and become shallower.
A gauge's zero point is chosen so that it does not move when the bed does. The US Geological Survey usually places it below the bed for that reason. So the reading does not follow the bed up or down, and it cannot show the depth.
An example in words: a gauge reads the same on two days a year apart. If the channel has silted up in between, the water is shallower on the second day, although the reading is the same.
What depth needs
The depth at a spot is the height of the water surface minus the height of the bed at that spot, both counted from the same reference. The surface height comes from the gauge. The bed height comes from a survey of the bed, which measures how far down the bed lies along and across the channel.
A survey shows the bed as it was on the day it was made. Where the bed moves, the depth is only known for as long as the survey still describes it.
World Water Levels holds no survey of any bed and shows no water depth. Nothing on a page of this site is a depth.
Lakes and reservoirs
On a lake or reservoir the number is the height of the water above sea level or another named datum, or the amount of water stored, as the source gives it. Neither is the depth of the lake at its deepest point or at the shore.
Boats and channels
How deep a channel is for a boat is a question for the waterway authority. For shipping, use the official notices of the waterway authority. This site gives readings, not channel depth.
Where this page comes from
The definition is the agency's own. Its page is named below, not linked. Search for its name to reach it.
- US Geological Survey, Why we use gage height
Changes
- : First published.